Aluminum shapes coated with brazing material and process of coating

ABSTRACT

An aluminum or aluminum alloy shape for manufacture of heat exchangers is coated with a metallic layer of a zinc-base alloy constituting a fluxless soldering or low temperature brazing material when heated. A process for direct coating of formed aluminum shapes in nascent state includes the steps of providing a non-reactive atmosphere around the shape, providing a coating material of zinc-base alloy and bringing the aluminum shape and the coating material into contact and maintaining the contact at an elevated temperature for a predetermined period of time for forming a coherent coating on the aluminum shape.

This application is a continuation application of Ser. No. 570,845,filed Jan. 16, 1984 now U.S. Pat. No. 4,615,952, which is acontinuation-in-part of application Ser. No. 437,884, filed Oct. 29,1982, now abandoned.

FIELD OF THE INVENTION

The present invention relates to coated aluminum shapes, and moreparticularly to hollow aluminum shapes, e.g. tubes coated with afluxless metallic layer, functioning as a soldering or low temperaturebrazing material when heated during assembling of the shapes to heatexchanging fins in the manufacture of heat exchangers, and to a processfor coating of such aluminum shapes.

It is to be understood that the word aluminum as used in the presentapplication is intended to mean aluminum and aluminum base alloys.

BACKGROUND OF THE INVENTION

Contact between tubes and heat exchanging fins in heatexchangers/radiators installed in cars is today ensured mainly by twomethods-mechanical expansion of tubes, or brazing of tubes to heatexchanging fins. The reliable contact achieved by the mechanicalexpansion of the tubes can be achieved only when using substantiallyround tubes, which means limited heat performance of the heatexchangers. Brazed flat copper or brass tubes are largely used in carradiators, providing a good performance when the inexpensive well-knownbrazing technique is used.

There is an increasing trend in the automotive industry to replace steeland copper with lighter materials like aluminum or magnesium.

Soldering and brazing of aluminum to aluminum or to other metals is,however, a difficult task because of the presence of an aluminum oxidelayer formed instantly on the surface of aluminum members exposed to theatmosphere. This oxide layer prevents formation of a tight metallic bondbetween brazed members because of its poor wetting properties.

In practice it is therefore necessary to pretreat the aluminum surfaceprior to soldering/brazing in order to remove the aluminum oxide.Different methods are used for removing the oxide layer, depending onthe subsequent use of the aluminum members. A method called flux brazingis widely used where the aluminum members are immersed in a bath ofmolten salt and brazing flux or exposed to the brazing flux whichremoves the oxide layer from the preheated members. The method is quiteefficient with regard to the oxide removal, but it is practicallyimpossible to avoid that the flux is entrapped at the brazed jointsrepresenting a serious corrosion problem.

Moreover, the brazing material commonly used is an Al-Si alloy which isa high temperature brazing alloy. In practice when the units to bebrazed are assembled with the parts in position on the coated tubes, theassembly is then placed in a vacuum furnace and heated. However, themelting temperature of this brazing alloy is around 1050° to 1100° F.which is not far below the melting point of aluminum, and as a resultthe temperature must be very closely controlled. But the silicon fromthe brazing alloy vaporizes and tends to deposit on surfaces within theheating apparatus, including the temperature sensors, thereby makingcontrol uncertain at best. Thus the process is not very reliable orefficient.

Fluxless brazing processes have been conducted successfully in vacuum orinert atmosphere and are described in U.S. Pat. Nos. 3,373,482 and3,979,042.

The common drawback of these processes is that the necessity ofcleaning/pretreating procedures is not eliminated and rather hightemperatures are required in order to break down the oxide layer or tocreate cracking in this layer. These processes are therefore expensiveand limited to special applications.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide coatedaluminum shapes, particularly hollow shapes, e.g. tubes having afluxless coating of soldering or brazing material for fluxlesssoldering/fluxless brazing at low temperatures, and to provide a processfor direct coating of aluminum shapes having an oxide-free,non-pretreated surface.

Another object of the invention is to provide aluminum shapes precoatedwith soldering or brazing material ensuring improved corrosionresistance of the shapes, such shapes being thus particularly suited forbrazed aluminum heat exchangers. By shapes is meant elongated aluminummembers having a cross-sectional shape such as an angle, an I or H beam,a flange, or any other of the many shapes in which elongated aluminummembers are formed, and particularly hollow shapes, e.g. tubes withvarious cross-sectional shapes-round, flat oval etc., optionallyprovided with inner partition walls or multicavities.

SUMMARY OF THE INVENTION

The aluminum shapes according to the present invention provided eitherby extruding the aluminum in the desired shape and dimension or by anyother methods of forming the shapes from aluminum are coated with ametallic layer which constitutes a fluxless soldering or brazingmaterial for the subsequent fluxless soldering or low temperaturebrazing of the shapes to other structural elements, e.g. soldering orbrazing of tubes to heat exchanging fins in the manufacture of heatexchangers. The soldering or brazing material applied as the coatinglayer consists of a zinc-base alloy.

The composite shape according to the invention is suitable for use inthe manufacture of heat exchangers, and comprises an aluminum oraluminum alloy core formed by a process in which the surface of the corewhich is formed is substantially free of oxide, and an outer layer offluxless soldering material of zinc or zinc base alloy which has beendeposited on the said aluminum core in a thickness sufficient for actingas a fluxless solder in a subsequent operation of joining the core toanother member, the deposition having been carried out with thesoldering material in a molten state while the metal of the core is in anascent state for causing a portion of the soldering material which istoward the core surface to diffuse into the aluminum core forinterbonding the soldering material to the aluminum core.

The process of the present invention comprises the steps of formingaluminum in the desired shape, preventing contact between the surface ofthe formed shape and the atmosphere for avoiding oxidation of thesurface prior to the subsequent coating of the outer surface with ametallic layer, and then carrying out coating of the outer surface ofthe shape with a metallic layer.

More particularly, the method of making the above-described compositeshape suitable for use in the manufacture of heat exchangers, comprisesforming an aluminum or aluminum alloy core by a process in which thecore surface in a nascent state is kept substantially free of oxide, anddepositing an outer layer of fluxless soldering material of zinc or zincbase alloy on said surface of the core in a molten state while the metalof said surface is in the nascent state, to cause the soldering materialto diffuse into the aluminum core for interbonding the solderingmaterial to the aluminum core, and the depositing being in a thicknesssufficient for acting as a fluxless solder in a subsequent operation ofjoining the core to another member.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in greater detail in connection withexamples and the accompanying drawing, FIG. 1, which is a schematicrepresentation of the process and the apparatus for carrying it out.

DETAILED DESCRIPTION OF THE INVENTION

A soldering/brazing coating provided on aluminum shapes according to thepresent invention is a zinc-base alloy, preferably ZnAl alloy,comprising from 1.0-10.0 wt % Al and from 0 to 3.0 wt % Cu, the balancebeing zinc. This alloy composition allows conducting of low temperaturecoating, approximately at 725°-790° F., and consequently low temperatureand low cost brazing of the coated tubes to heat exchanging fins duringassembling of heat exchangers. The addition of Al, preferentially from3.0 to 6.0 wt %, improves the wetting of aluminum by molten zinc and incombination with Cu results in a sufficiently ductile coating of hightensile strength of the shapes in cold state.

EXAMPLE 1

A drawn aluminum tube was mechanically cleaned of its surface oxides ina protective atmosphere of nitrogen and then passed directly, i.e. whilestill in the nitrogen protective atomosphere, into a molten bath of zincalloy comprising 5.8 wt % Al, 1.0 wt % Cu and the balance mainly zinc.The zinc bath temperature was kept at 840° F. in order to avoid anexcessive solidification of zinc on cold aluminum. After a 3 sec.contact time the tube was removed from the zinc bath and then cooled bywater spray. A coherent layer of zinc coating was achieved on the thustreated drawn aluminum bath.

The apparatus for carrying out the particular coating method accordingto the present invention as shown in FIG. 1, comprises a formingapparatus, e.g. an extruder (1), for forming an aluminum member (2) in adesired form, expecially that of a tube, from aluminum (3) suppliedthereto. Surrounding the exit from the extrusion die (4) of the extruderand in substantially gas-tight relation with the end of the extruderfrom which the die extends, is an oxidation preventing chamber (5) towhich an oxidizing preventing gas is supplied through an inlet (6), e.g.an inert gas such as nitrogen. It is sufficiently short so that theextruded shape does not cool below the desired coating temperaturebefore it passes out of the chamber. From the oxidation preventingchamber the extruded aluminum shape is passed into a means for coatingthe shape, e.g. a bath (7) of soldering or brazing metal without beingexposed to the atmosphere, i.e. so that it is prevented from beingoxidized. In a specific embodiment of the apparatus of the invention,this is done by attaching the bath directly to the outlet end of theoxidation preventing chamber (5). In this specific embodiment the thuscoated aluminum shape can then be passed through a conventional wiper(8) to wipe excess coating metal therefrom, and then cooled, either bybeing cooled in the ambient atmosphere or passed through a cooling meanssuch as a water quenching apparatus (not shown).

This means for applying the coating is here shown as a bath (7) throughwhich the aluminum shape (2) is directly passed. However, a variety ofknown coating processes such as dipping, spray-coating, and flashvaporization can be used within the scope of the present invention. Caremust be taken that the extruded shape is not exposed to the ambientatmosphere or other oxidizing atmosphere between the time it leaves theextruder and the time it is coated.

The extruded shapes in status nascendi, i.e. the nascent state, afterleaving the extrusion die are free of oxide on all surfaces and remainat a temperature of approximately 750°-800° F. so that an additionalheating of the shapes is not required. The resulting coated aluminumshapes offer a corrosion resistant, leak-proof tubing particularlysuited for heat exchangers.

A further advantageous characteristic which is given by the presentmethod to the extruded shapes as compared with conventional coatingmethods is improved workability. In the prior art process when theextruded aluminum shape is preheated prior to application of brazingflux or immersed in a bath of molten salt and brazing flux in order toremove the oxide coating the aluminum is heated sufficiently, whichcauses grain growth in the aluminum, thereby reducing the workability.In the method of the present invention, since coating takes placeimmediately after extrusion without an intermediate heating step, thereis insufficient time for any significant grain growth.

EXAMPLE 2

A hollow aluminum tube having a flat, oval cross-section is extrudedfrom a conventional extruder at a temperature of about 950° F., and ispassed directly, i.e. without being exposed to the ambient atmosphere,into an oxide preventing chamber connected in substantially gas-tightrelation to said extruder around the extrusion die thereof. The chamberis filled with nitrogen and is about 70-100 cm long. At the end of thechamber the tube has cooled to about 750° F., an excellent temperaturefor applying a soldering or brazing material thereto. The thus cooledtube is then passed directly, i.e. without being exposed to the ambientatmosphere, into a bath of a Zn-Al fluxless soldering alloy composed ofAl in an amount of about 3.5 to 4.5 wt %, Cu in an amount of about 2.5to 3.5 wt %, Mg about 0.05 wt % and the balance Zn, which is at atemperature of from 715°-720° F.

Thereafter the thus coated tube is passed through a conventional wiperin which the excess coating metal is wiped from the tube, and then thetube is cooled in the atmosphere.

The thus completed tube can then be easily fabricated into a structureby having pieces soldered thereto. For example, where the tube is toform the core tube of a radiator, the fins can be positioned on the tubeand the tube simply passed through a heating chamber at about 750° F.,whereby the fins will be soldered the tube. No soldering flux need to beused and the soldering temperature is very low compared to the prior artwhich means low cost assembling of the radiators.

I claim:
 1. A composite shape suitable for use in the manufacture ofheat exchangers, said shape comprising an aluminum or aluminum alloycore formed by a process in which the surface of the core which isformed is substantially free of oxide, and an outer layer of fluxlesssoldering material of zinc or zinc base alloy which has been depositedon the said aluminum core in a thickness sufficient for acting as afluxless solder in a subsequent operation of joining the core to anothermember, the deposition having been carried out with the solderingmaterial in a molten state while the metal of the core is in a nascentstate such that a portion only of the soldering material which is towardthe core surface is diffused into the aluminum core, therebyinterbonding the soldering material to the aluminum core.
 2. A compositeshape as claimed in claim 8, wherein said core is an extruded hollowshape.
 3. A composite shape as claimed in claim 8, wherein said core isa drawn hollow shape which has been subsequently heated up to atemperature equal to or higher than the melting point of the solderingmaterial.
 4. A composite shape as claimed in claim 1, wherein the entiresurface of said core is covered by the soldering material.
 5. A methodof making a composite shape suitable for use in the manufacture of heatexchangers, said method comprising forming an aluminum or aluminum alloycore by a process in which the core surface in a nascent state is keptsubstantially free of oxide, and depositing an outer layer of fluxlesssoldering material of zinc or zinc base alloy on said surface of thecore in a molten state while the metal of said surface is in the nascentstate, to cause a portion the soldering material to diffuse into thealuminum core for interbonding the soldering material to the aluminumcore, and the depositing being in a thickness sufficient for acting as afluxless solder in a subsequent operation of joining the core to anothermember.
 6. A method as claimed in claim 5, wherein said forming of thecore is by a continuous forming process, and said outer layer is formedby a continuous deposition process in line with said forming process. 7.A method as claimed in claim 6, wherein said forming of the corecomprises extruding a hollow shape.
 8. A method as claimed in claim 6,wherein said forming of the core comprises drawing a hollow shape andsubsequently heating it up to a temperature equal to or higher than themelting point of the soldering material.
 9. A method as claimed in claim5, wherein the soldering material is deposited at a temperature abovethe melting point of the soldering material and up to 840° F.
 10. Amethod as claimed in claim 5, wherein said depositing comprisesdeposition soldering material on the entire suface of said core.